This application claims benefit of priority under 35USC xc2xa7119 to Japanese Patent Application No. 2000-127087 filed on Apr. 27, 2000 and Japanese Patent Application No. 2000-364648 filed on Nov. 30, 2000, the entire contents of which are incorporated by reference herein.
1. Field of the Invention
The present invention relates generally to a frequency multiplier and semiconductor integrated circuit for producing a local oscillation signal for use in a superheterodyne receiver or the like.
2. Related Background Art
Recently, there are provided various systems for sending and receiving weak radio waves to carry out various processes with non-contact. For example, a keyless entry system is designed to receive weak radio waves emitted from a transmitting circuit embedded in a key for a vehicle, by a receiving circuit in the vehicle to open and close doors and so forth.
FIG. 19 is a block diagram showing schematic configuration of a conventional weak radio wave sending/receiving system of this type. The system of FIG. 19 generally comprises a transmitter 51 and a receiver 52. The transmitter 51 has a transmitting circuit 53 and an antenna 54. The transmitter 51 uses a carrier frequency of 315 MHz to emit AM-modulated (amplitude-modulated) or FM-modulated (frequency-modulated) signals via the antenna 54.
The receiver 52 comprises an antenna 11, an SAW filter 12, an RF amplifier 13, a local oscillator circuit for generating a local oscillation signal, a mixer 15 for generating an intermediate frequency signal (IF signal), an IF filter 16, an IF amplifier 17 and a detector circuit 18. The local oscillator circuit 14 has a source oscillator circuit 21 for generating a reference signal, and a quintupler circuit 20 for outputting a quintupled signal having a frequency five times as high as that of the reference signal.
The source oscillator circuit 21 is designed to generate a source oscillation signal having a frequency of 65.14 MHz. The local oscillator circuit 14 is designed to generate a local oscillator signal fLO=325.7 MHz which has an increased frequency five times as high as the frequency of the source oscillation signal. The mixer 15 is designed to use the local oscillation signal fLO to output an intermediate frequency signal having a frequency of fLOxe2x88x92fO=10.7 MHz.
Thus, a high frequency signal received by the antenna 11 is converted into an intermediate frequency signal by the mixer 15, so that the signal processing can be more easily carried out than a case of performing the signal processing by directly using the high frequency signal.
The IF filter 16, which is a band-pass filter, is connected to the subsequent stage mixer 15. The passing band of the filter 16 is about hundreds kHz centering on an intermediate frequency of 10.7 MHz. After undesired frequency components are removed by the IF filter 16, the intermediate frequency signals of 10.7 MHz are amplified by about 70 dB in the IF amplifier 17.
When the system of FIG. 19 is applied to the above described keyless entry system, the transmitter 51 is embedded in a key carried by a human, and the receiver 52 is mounted on a vehicle. From the transmitter 51, weak radio waves having the frequency of 322 MHz or less are emitted. An allowable field intensity is 500 xcexcV/m or less which is defined by Article 6 of Enforcement Regulations of Radio Wave Law in Japan. Radio waves having the frequency of 322MHz or higher can be used. However, the allowable field intensity from 322 MHz to 10 GHz is 35 xcexcV/m or less which is very small. In addition, as the frequency increases, the progressivity of radio waves increases so as not to be put to practical use, so that the radio waves having a frequency of 322 MHz or higher are hardly used in the country. Therefore, radio waves having a frequency of about 315 MHz are generally used as weak radio waves.
On the other hand, the transmitter 51 preferably has smaller electric power consumption in order to increase the life of a battery, so that it is required to simplify the circuit construction. For example, an SAW vibrator is known as a simple element capable of oscillating radio waves having a frequency of about 315 MHz. This element can not only simplify the circuit construction, but it can also directly oscillate by a frequency of 315 MHz.
Although there is a crystal oscillator as another oscillator element, it is technically difficult to directly oscillate the signal having a frequency of about 315 MHz. After oscillating the crystal oscillator at a low frequency, it is required to multiply frequency. Because of this, the SAW vibrator is often used to constitute the circuit as simple as possible.
However, there is a problem in that the SAW vibrator has a large frequency deviation. The frequency deviation of the SAW vibrator is usually 100 ppm or higher. The frequency deviation of the transmitter itself grows worse if the SAW vibrator is used as the transmitter. Because of this, when the performance and yields of products are intended to be improved, a crystal oscillator is sometimes used.
When the SAW vibrator is used as the transmitter, it is required to improve the frequency precision on the side of the receiver in order to compensate its disadvantages. In order to improve the frequency precision, a crystal oscillator having a small frequency deviation is generally used for the local oscillator circuit of the receiver. Since the frequency deviation of the crystal oscillator hardly exceeds 100 ppm at worst, it is possible to improve the frequency precision of the receiver by using the crystal oscillator for the local oscillator circuit in the receiver.
However, since it is very difficult to directly oscillate a high frequency of 300 MHz band as described above, a method for lowering the oscillation frequency of the crystal oscillator itself to multiply frequency by a frequency multiplier circuit to obtain a frequency of 300 MHz band is generally used.
There is proposed a technique using, as a conventional oscillator circuit, a quintupler circuit 20 for oscillating a crystal oscillator at 65.14 MHz to generate higher harmonics by distorting its waveform and extracting fifth-order higher harmonics by means of a filter or the like to obtain a frequency of 325.7 MHz when a local oscillation frequency of, e.g., 325.7 MHz (=315+10.7 MHz) is intended to be generated.
In this quintupler circuit 20, it is required to increase distortion to generate higher-order higher harmonics in order to increase the multiple number. Although the level of higher harmonic components decreases as the order thereof increases, the higher harmonic components include many undesired components other than originally required fifth-order higher harmonics.
Now, assuming that a rectangular wave of an even function is used as a distorted wave and assuming that it has xe2x80x9c1xe2x80x9d in an interval of from (xe2x88x92x) to (+x) and xe2x80x9cxe2x88x921xe2x80x9d in other intervals, the following expression can be expressed:
A(xxe2x88x92xcfx80/2)+Asin xxc2x7cos xcfx89t+A/2 sin 2xxc2x7cos 2xcfx89t+A/3xc2x7sin 3xxc2x7cos 3xcfx89t+. . . +A/nxc2x7sin nxxc2x7cos xcfx89txe2x80x83xe2x80x83(1)
wherein A is a constant, xcfx89 is an angular frequency which is 2xcfx80fLO, t is time, n is a natural number, and the first term is a dc component.
In expression (1), for example, assuming that n=5, the fifth-order higher harmonic components attenuate to ⅕ as large as a fundamental wave. Assuming that x=xcfx80/2, the odd orders of expression (1) remain, and the dc component is zero only in this case.
In order to utilize only the fifth-order higher harmonic components, first through fourth order components and sixth or higher order components must be removed. Therefore, only the fifth-order higher harmonic components are extracted by a filter using the quintupler circuit 20. However, undesired higher harmonic components remain at a high level. The undesired higher harmonic components are not only propagated in a space, but the frequencies are also close to each other, so that it is require to use an SAW filter or the like.
If the undesired higher harmonic components come into the mixer 15 via the space and a transmission line, the higher harmonics themselves function as jamming waves. In addition, all of radio waves having a frequency foxe2x80x2 wherein the difference between the frequency fLOxe2x80x2, and the frequency foxe2x80x2 of undesired radio waves coming from the outside is equal to an intermediate frequency of 10.7 MHz function as harmful radio waves.
If the receiver is influenced by the harmful radio waves, the keyless entry system may not normally operate, so that it is required to decrease the influence of undesired higher harmonics.
For example, in order to decrease the influence of higher harmonics in the prior art, it is required to take measures to provide a shielding for the frequency multiplier circuit or to spatially increase the distance between the frequency multiplier circuit and the mixer.
Since it is required to prevent the multiple number from increasing in order to reduce the higher harmonics, there is a limit to the multiple number, so that there is a problem in that the oscillation frequency can not be so low. If the source oscillation frequency is high, it is difficult to design the circuit, and the circuit is complicated. In addition, the cost of providing the crystal oscillator is also high.
In order to lower the source oscillation frequency, it is considered to use a phase locked loop (PLL) circuit. However, if the PLL circuit is used, it is required to use an oscillator circuit and a voltage control oscillator (VCO) circuit for obtaining a phase-comparison frequency. In addition, the circuit scale increases, and the costs increases by the PLL circuit and the voltage control oscillator circuit.
Because of this, it is considered that the PLL circuit and the voltage control oscillator circuit are included in one chip to form as an IC and to suppress the increase of the circuit scale and costs. However, if the voltage control oscillator circuit is included in the chip, the C/N ratio deteriorates, so that performance such as sensitivity deteriorates. In order to prevent this, the voltage control oscillator circuit must be provided outside, so that it is not possible to reduce the circuit scale when performance is regarded as important.
In order to combine the PLL circuit with the voltage control oscillator circuit, higher harmonics called phase-comparison spurious occurs, so that it is required to take measures to remove the higher harmonics.
Thus, the conventional oscillator circuit is designed to oscillate at a low frequency using the crystal oscillator in order to improve the frequency precision of the receiver and to quintuple the oscillation frequency of the crystal oscillator by the frequency multiplier circuit, and is provided with a shielding or the like in order to decrease generated undesired higher harmonic components. Therefore, there is a limit to the miniaturization of the circuit, and there is a problem in that it is required to take measures to remove jamming waves.
It is therefore an object of the present invention to eliminate the aforementioned problems and to provide a frequency multiplier circuit and semiconductor integrated circuit capable of surely removing undesired frequency components with a simple circuit construction.
In order to accomplish the aforementioned and other objects, according to one aspect of the present invention, a frequency multiplier circuit comprises: a source oscillator configured to generate a source oscillator signal using a crystal oscillator; and n frequency multiplier circuits (n is an integer which is 2 or more), each of which includes a 90xc2x0 phase shifter circuit configured to shift the phase of an input signal by 90xc2x0, and a mixer configured to generate a doubled signal of the input signal on the basis of the input signal and an output signal of the 90xc2x0 phase shifter circuit, wherein the n frequency multiplier circuits are cascade-connected, the source oscillation signal being inputted to a first stage frequency multiplier circuit of the n frequency multiplier circuits, and a final stage frequency multiplier circuit of the n frequency multiplier circuits outputting a signal having a frequency 2n times as high as the frequency of the source oscillation signal.
According to another aspect of the present invention, a semiconductor integrated circuit comprises: a local oscillator circuit configured to generate a local oscillation signal; an intermediate frequency signal converter configured to convert a high frequency signal, which is received by an antenna, into an intermediate frequency signal on the basis of the local oscillation signal; and a demodulator configured to carry out a demodulation processing on the basis of the intermediate frequency signal, wherein the local oscillator circuit comprises: a source oscillator configured to generate a source oscillation signal using a crystal oscillator; and n frequency multiplier circuits, each of which includes a 90xc2x0 phase shifter circuit configured to shift the phase of an input signal by 90xc2x0, and a mixer configured to output a doubled signal of the input signal on the basis of the input signal and an output signal of the 90xc2x0 phase shifter circuit, the n frequency multiplier circuits being cascade-connected, and the source oscillation signal being inputted to a first stage frequency multiplier circuit of the n frequency multiplier circuits.
According to the present invention, since the n frequency multiplier circuits are cascade-connected to supply the source oscillation signal from the crystal oscillator to the first stage frequency multiplier circuit, it is possible to output a sufficiently high frequency signal from the final stage frequency multiplier circuit even if the source oscillation frequency is low. Therefore, it is possible to easily design the source oscillator, and it is possible to stabilize the characteristics of the source oscillation signal.
In addition, since the frequency multiplier circuit comprises the 90xc2x0 phase shifter circuit and the mixer, it is possible to realize a frequency multiplier circuit which can efficiently suppress undesired frequency components and which is resistant to jamming. It is also possible to obtain a sufficiently high frequency signal by adjusting the number of stages of connected frequency multiplier circuits.
If the frequency multiplier circuit is provided with a filter circuit, it is possible to surely remove undesired high frequency components. If a variable impedance element is provided in the filter circuit, it is possible to control the frequency characteristics of the filter circuit in accordance with the phase shift quantity of the phase shifter circuit.
If both of the filter circuit and the phase shifter circuit are provided with a limiter amplifier, it is possible to prevent the frequency characteristics of the filter circuit and the cut-off frequency of the phase shifter circuit from depending on the input signal level.
If an impedance element is connected to the variable resistor of the filter circuit in series, it is possible to cancel the fixed phase shift quantity caused by the limiter amplifier of the phase shifter circuit, and it is possible to control the frequency characteristics of the filter circuit so as to faithfully follow the variation in source oscillation frequency.
If the frequency multiplier circuit according to the present invention is used for forming a superheterodyne receiver, portions other than the crystal oscillator and the antenna are provided in one chip, so that it is possible to miniaturize the receiver and reduce costs.